Reassessing the 1987 Parraguirre Ice-Rock Avalanche in the Semi-Arid Andes of Chile

Chile faces high vulnerability to mountain hazards along the Andean Cordillera. As climate change and urban development intensify, the frequency and impact of destructive debris flows are anticipated to rise. To inform mitigation and adaptation strategies, it is imperative to understand the characteristics of historical events in this region. A notable example is the Parraguirre rock avalanche that occurred on November 29, 1987, which transformed into a catastrophic debris flow, travelling 50 kilometers down-valley and causing severe damage and loss of human lives. The high destructive power is attributed to the considerable amount of water involved. Yet, the source of this water remains largely unidentified - so is the initial trigger volume and the total mass transfer down the valley.

In this study, we revisit the past event by integrating new insights from remote sensing, climate and hydrological records as well as process-based modelling. Our results suggest important corrections. We find a trigger volume of 17.0±1.4·10⁶ m³ and a total fluid flood volume of 16.0·10⁶ m³. The solid mass transfer from the Parraguirre catchment amounts to 38.1±15.2·10⁶ m³. Moreover, we find that the elevated water content cannot be solely attributed to the entrainment of soil moisture and snow cover. It requires a considerable contribution from another source - likely in form of glacier ice. Furthermore, our simulations corroborate the damming hypothesis of Río Colorado, thereby reconciling the observations of multiple waves as well as on arrival times and run-out distance.

Apart from the geological and tectonic preconditions, we propose to classify the Parraguirre rock avalanche as a meteorological compound event. This classification is motivated by the exceptionally high snowpack observed in the spring of 1987, which preconditioned elevated snowmelt rates during a series of unusually warm days at the end of November. Such preconditioning factors are readily accountable in monitoring efforts and early-warning systems for such mountain hazards.